B lymphopoiesis is severely compromised in murine senescence. While there are defects at several distinct stages in the B lymphopoietic pathway in old age, down-regulation at the pro-B to pre-B cell transition likely has a major role in altering the composition of the B cell antibody repertoire. This transition is particularly dependent upon expression of and signaling by the pre-B cell receptor (preBCR), a complex of immunoglobulin 5 heavy chain associated with the surrogate light chains l5 and VpreB. Aged mice have significantly reduced expression of surrogate light chains and poor preBCR function. This results in progressive alteration of the 5 heavy chain repertoire in newly formed B cells in aged mice. However, rather than a random loss of pre-B cells in old age, we propose that the aged pre-B compartment will become skewed in favor of those pre-B cells that undergo positive selection even when surrogate light chain is highly reduced. Moreover, we propose that increases in autoreactivity as well as detrimental changes in the availability of protective antibody responses to pathogens, e.g., S. pneumoniae, are, in part, due to a compromised preBCR checkpoint. To address this hypothesis, we propose 3 integrated Specific Aims. In Specific Aim 1, we ask Does compromise of the preBCR checkpoint in old age 'reshape' the Vh repertoire and promote autoreactivity? Here, the effects of diminished preBCR function on 5 heavy chain usage, and capacity to signal when surrogate light chain is low, will be determined together with impact on autoreactivity. Specific Aim 2 addresses the fate of autoreactive immature B cells within the bone marrow of aged mice and their contribution to the peripheral B cell pools. Tolerance among immature B cells in aged mice, their capacity to home to the spleen, and, importantly, the expression of protective versus non-protective clonotypes in response to the S. pneumoniae antigen phosphorylcholine will be assessed. Specific Aim 3 focuses upon the importance of proinflammatory cytokines (TNFa) and effector cells (NK) in triggering the down-regulation of preBCR, altering both B lymphopoiesis and antibody repertoires in aged mice. These studies will advance understanding of the immune defects that accompany old age and their cellular and molecular mechanisms.